Scientists calculate radiation release from Fukushima

Measuring radioactive sulfur in California let researchers calculate how much radiation leaked from the damaged nuclear reactors in Fukushima, Japan.

Atmospheric chemists say they now have the first quantitative measurement of the amount of radiation leaked from the damaged nuclear reactors in Fukushima, Japan following the March, 2011 earthquake and tsunami there. They used direct measurements of radioactive sulfur in California – taken in late March and early April – to calculate how much radiation must have leaked from the Fukushima reactors.

These researchers continuously monitor atmospheric sulfur in San Diego, using an instrument at the end of a pier at the University of California San Diego’s Scripps Institution of Oceanography. They were able to use these measurements to calculate that concentrations of radioactive materials a kilometer or so above the ocean near Fukushima must have been about 365 times higher than natural levels.

Mark Thiemens is dean of the Division of Physical Sciences at University of California at San Diego and senior author of the paper on this subject this week (August 15, 2011) in the early, online edition of the Proceedings of the National Academy of Sciences. He told EarthSky there was no danger to people in California from radioactive sulfur measured by his team in March and April:

Although the spike that we measured was very high compared to background levels of radioactive sulfur, the absolute amount of radiation that reached California was small. The levels we recorded aren’t a concern for human health. In fact, it took sensitive instruments, measuring radioactive decay for hours after lengthy collection of the particles, to precisely measure the amount of radiation.

On March 28, 2011, 15 days after operators began pumping seawater into the damaged reactors and pools holding spent fuel, Thiemens’ group observed an unprecedented spike in the amount of radioactive sulfur in the air in La Jolla, California. They recognized that the signal came from the crippled power plant.

Neutrons and other products of the nuclear reaction leak from fuel rods when the rods melt. Seawater pumped into the reactor absorbed those neutrons, which collided with chloride ions in the saltwater. Each collision knocked a proton out of the nucleus of a chloride atom, transforming the atom to a radioactive form of sulfur.

When the water hit the hot reactors, nearly all of it vaporized into steam. To prevent explosions of the accumulating hydrogen, operators vented the steam, along with the radioactive sulfur, into the atmosphere.

In air, sulfur reacts with oxygen to form sulfur dioxide gas and then sulfate particles. Both blew across the Pacific Ocean on prevailing westerly winds to the monitoring equipment in San Diego maintained by Thiemens’ group.

Using a model based on NOAA’s observations of atmospheric conditions, the team determined the path air took on its way to the pier over the preceding 10 days, and found that it led back to Fukushima.

Then they calculated how much radiation must have been released.

Antra Priyadarshi, a post-doctoral researcher in Thiemens’ lab and first author of the paper, said:

You know how much seawater they used, how far neutrons will penetrate into the seawater and the size of the chloride ion. From that you can calculate how many neutrons must have reacted with chlorine to make radioactive sulfur.

After accounting for losses along the way as the sulfate particles fell into the ocean, decayed, or eddied away from the stream of air heading toward California, the researchers calculated that 400 billion neutrons were released per square meter surface of the cooling pools between March 13, 2011, when the seawater pumping operation began, and March 20.

Concentrations a kilometer or so above the ocean near Fukushima must have been about 365 times higher than natural levels to account for the levels the researchers observed in California.

The radioactive sulfur that Thiemens and his team observed must have been produced by partly melted nuclear fuel in the reactors or storage ponds. Although cosmic rays can produce radioactive sulfur in the upper atmosphere, it rarely mixes down into the layer of air just above the ocean, where these measurements were made.

Over a four-day period ending on March 28, they measured 1501 atoms of radioactive sulfur in sulfate particles per cubic meter of air, the highest they’ve ever seen in more than two years of recordings at the site.

Even intrusions from the stratosphere – rare events that bring naturally produced radioactive sulfur toward the Earth’s surface – have produced spikes of only 950 atoms per cubic meter of air at this site.

Bottom line: Atmospheric chemists at University of California San Diego – including Mark Thiemens and Antra Priyadarshi – directly measured radioactive sulfur in California in late March and early April 2011. They then used those measurements to calculate how much radiation must have leaked from the damaged nuclear reactors in Fukushima, Japan following the devastating earthquake and tsunami there in March 2011. They say their work represents the first quantitative measurement of the amount of radiation leaked from the damaged Japanese nuclear reactors. They calculated that concentrations of radioactive materials a kilometer above the ocean near Fukushima must have been about 365 times higher than natural levels after the meltdown. The levels measured in California were not high enough to harm people, according to these researchers.

Deborah Byrd created the EarthSky radio series in 1991 and founded the website EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website and blogs frequently about astrophysics, the night sky and other topics related to Earth, space and the human world. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.